As semiconductor circuits become more complex, increasingly stringent requirements are placed on their flexibility. It is, therefore, necessary to provide such semiconductor circuits with programmable memories that can be programmed after manufacture of the integrated circuit (IC). Erasable programmable read-only memories (EPROMs) can be used for this purpose. Programmable circuit structures with programmable elements, such as EPROMs or gate regions, are well known in the art. One such programmable circuit is disclosed in WO 92/20095. This reference describes programmable elements which include an antifuse region. The antifuse region is made of a material which is not electrically conductive in a first state, the unprogrammed or high-impedance state, but which is made conductive in a second state, the programmed or low-impedance state, by applying a sufficiently high voltage thereto. The antifuse region described in WO 92/20095 is used to short-circuit two conductors by applying an external voltage.
There are, however, some disadvantages associated with the use of EPROMs, such as the one described in WO 92/20095. One such disadvantage is that EPROMs are relatively complicated and costly to fabricate. Another disadvantage is that EPROM circuits take up relatively more space than other types of semiconductor circuits.
It is, therefore, the object of the present invention to provide a programmable semiconductor memory which can be fabricated with an MOS process of low complexity and takes up less space than prior art programmable semiconductor memories.